Before knowing how to monetize, we need to know why people willing to pay a price to use your platform/ service.

If you decided to try out different monetize approaches without first find out the above, I guarantee a very low success rate, your success will be due to luck rather than a controllable approach.

Suggest you first conduct a deep analysis of your directory list to understand:
1) the group that most insensitive when you charge a price
2) find out the reason why they are insensitive
3) set a reasonable pricing model and start monetizing

If you only have a very small number of members on the list, your above test will be less accurate.

Upon getting to step 3 above, you can try different monetizing methods to find the one that is most appropriate for your business model.